1. Field of the Invention
This invention relates to a filtered venting and decay heat removing apparatus and system for a nuclear reactor containment and the method of operation thereof and, more particularly, to such apparatus, system and method for both filtering and cooling and also removing decay heat of the mixture of gases, particulates and aerosols vented from a nuclear reactor containment during severe accident sequences, to avoid late containment over-pressure failure and subsequent uncontrolled fission product release.
2. State of the Prior Art
The containment of a nuclear reactor is designed to withstand a predetermined pressure level, termed the pressure level set point, below which it will contain and thus prevent uncontrolled fission product release to the atmosphere in the event of a postulated major accident. Extensive analyses of the postulated accident scenarios applicable to a given reactor installation are performed for determining, or projecting the elapsed time interval sequences over which major events occur. In the case of a LOCA (loss of coolant accident), projections are made as to a first time interval over which the decreasing level of coolant within the reactor vessel causes the top of the core, i.e., the fuel element assemblies, to become exposed, a second time interval over which the fuel element assemblies start to melt, and a third time interval during which pressurizing increases to the pressure level set point of the design basis for the containment--beyond which containment failure due to over-pressure will occur, accompanied by the uncontrolled release of fission products to the atmosphere.
Accordingly, to protect against such late containment over-pressure failures, it has been proposed to provide reactor installations with appropriate systems and apparatus to achieve controlled and filtered venting of the containment in the event of a postulated major accident. The vent flow from the containment is a mixture of gases, particulates and aerosols, typically at high pressure and temperature, and since containing fission products, presenting the further problem of decay heat. For convenience, this vented flow is referred to as the gases vented from the containment and will be understood to include the mixture as before-stated. Stringent standards are set with regard to the percentage, of the vented gases which may be released to the atmosphere, as a time-integral function. For example, some countries have established a minimum removal efficiency of 99.9% for a specific type of a medium-size reactor, and an even greater percentage efficiency typically is required for larger reactors. The time-integral function, as relates to the controlled venting, typically postulates a time interval of 24 hours during which the system must function passively and automatically in response to the accident, following which it is assumed that further remedial activities may be undertaken by emergency maintenance and repair crews. In addition to satisfying these removal efficiency standards, hydrogen gas frequently is produced, which may require separate handling in view of the threat of hydrogen combustion which it presents. Conventional, alternative approaches include properly confining it and disposing of it by controlled ignition and venting of the combustion products to the atmosphere, or if levels are not excessive, use of a sufficiently large chimney or stack to vent it to the atmosphere. More sophisticated techniques involve rendering it inert with nitrogen, which requires sealing of the vessel into which the vented gases flow to preserve the nitrogen level and subsequent controls for thereafter disposing of the nitrogen, such as by the ways just mentioned, with replenishment of the nitrogen for maintaining the inert environment and preventing combustion.
Various techniques and apparatus have been proposed for the purpose of scrubbing, or filtering, the gases vented from a containment facility. Two basic types of such apparatus include pool-type scrubbers and a sand or gravel filters, the respective characteristics and features of which are described in U.S. Pat. No. 4,432,777--Postma, assigned to the United States of America as represented by the United States Department of Energy and entitled "METHOD FOR REMOVING PARTICULATE MATTER FROM A GAS STREAM."
The Postma patent discloses a so-called "hybrid scrubber" comprising a porous bed which is at least partially submerged within a pool of liquid and is characterized as merging the desirable features of both a pool-type scrubber and a sand or gravel filter. FIG. 1 of that patent is reproduced as FIG. 1 herein, and substantial portions of the description of the hybrid scrubber in that patent are now set forth. As described beginning at column 2, line 62 of the patent specification, the Postma hybrid scrubber: "comprises a liquid-tight enclosure 10 having a bottom wall 11 and connecting upright side walls 12 defining an interior liquid tank. The tank is preferably fully enclosed and completed by a top wall 13, but can be upwardly open, as will be described below.
"A quantity of liquid 14 is contained within enclosure 10. This liquid might be water of any desired liquid that is physically stable and compatible with the structure and usage of the filter. It partially fills the interior liquid tank presented by enclosure 10 to a liquid surface elevation designated by reference numeral 15.
"An open ended container 16 is positioned within enclosure 10. It has gas impervious upright side walls 17 that extend from a lower end 18 to an upper end 20. The lower end of container 16 is submerged in the liquid 14. Its upper end 20 is either adjacent to, above or below the liquid surface elevation shown at 15.
"A porous bed 21 of gravel or other filtering material is surrounded by the gas impervious side walls 17 of container 16. Bed 21 extends vertically upward within container 16 from a bottom location spaced above the lower end 18 of the gas impervious upright side walls 17 of container 16. This location is defined by a transverse porous or perforated plate 22 extending across the side walls 17. In the preferred embodiment as shown, approximately half of the vertical height of the porous bed 21 is located beneath the elevation of the liquid surface at 15 and is therefore submerged in the liquid 14.
"An inlet duct 23 is provided for directing a stream of pressurized gas and particulate material or aerosol to a submerged location vertically beneath the porous bed 21. This is illustrated as a vertical tube made of gas impervious material and extending through the center of the porous bed 21. The inlet duct 23 terminates at an open bottom end 24 positioned at an elevation between the bottom of porous bed 21 and the lower end 18 of the gas impervious container walls 17.
"The top end of the porous bed 21 is illustrated as being covered by a transverse porous or perforated plate 25. While such a plate is desirable, it is not always necessary to keep bed 21 confined. The top end of bed 21 is transversely open to liquid flow to thereby permit liquid entrained with the stream of gas to be returned by gravity over the sides of container 16 to the liquid 14 within the interior liquid tank provided by enclosure 10.
"Various materials might be used within porous bed 21. The porous material should be insoluble in the liquid. It might constitute natural or artificial sand or gravel, fibrous materials, or other packing materials commonly used in either dry or wet filters.
"An outlet duct 26 is open through enclosure 10 at an elevation above the liquid surface at 15. Duct 26 discharges the stream of gas following its passage through the porous bed 21.
"The presence of the gas within the container 16 that surrounds the porous bed 21 reduces the apparent density of the liquid 14 within the bed confines. Consequently, as the gaseous stream rises through bed 21, liquid flows from within enclosure 10 into the bottom of the bed, moves upwardly, and subsequently spills over the top. Collected aerosol within porous bed 21 is thereby continuously washed from it. This passive, self-cleaning function of the porous bed 21 is one of the novel features of this device.
"The illustrated apparatus effectively moves aerosols from a gaseous stream. The efficiency of aerosol removal can be adjusted by modifying the depth of the porous bed 21, the size of the packing materials comprising bed 21 and the velocity of the gaseous stream directed through the inlet duct 23 and bed 21. The only limitation as to the amount of collected material which can be accommodated by the apparatus is the volume of the pool of liquid 14 and the solubility of the removed aerosol materials within the liquid. Another limit is the volume of insoluble particles that can be accommodated within the enclosure 10.
"The scrubber is a three phase liquid scrubber. The solid phase, comprising the porous material within the bed 21, is fixed in place. The gas and liquid phases flow concurrently through bed 21. Because of the complexity of such a system, tests were conducted to both develop the concept and measure scrubber performance.
"A prototype scrubber was constructed substantially as shown in FIG. 1. The bed was 0.30 m in diameter, 0.61 m deep, and was packed with basalt rock sieved to between +0.91 cm and -1.27 cm. The cross-sectional area available for gas flow was 0.069 m.sup.2. The bed void fraction was 0.450+/-0.050.
"The granular basalt rock used in these tests is characterized as having no smooth sides. It was screened by hand into three segments. It was retained between horizontal plates 22 and 25 across the container side walls 17. Plates 22 and 25 as tested were made from solid flat sheets with apertures formed through them in a staggered pattern and a central aperture to receive the inlet duct 23. A second type of support plate usable in this apparatus could be fabricated from suitable screen material.
"It is to be noted that the lower end 18 of the container side walls 17 is provided with openings 27. They are spaced above the bottom wall 11 of enclosure 10 to prevent reentrainment of insoluble solids within the liquid and gas stream moving into porous bed 21. The openings 27 permit flow of liquid 14 beneath the porous bed 21. The areas between the openings 27 and the bottom of bed 21 constitutes a surrounding skirt within which incoming gas briefly accumulates before it moves upwardly through the porous bed 21.
"As is evident from FIG. 1, the horizontal cross-sectional area of container 16 is substantially less than the interior horizontal cross-sectional area within the enclosure 10. The cross-sectional area of container 16 is a function of the volume of gas which must be passed upwardly through bed 21. The cross-sectional area and depth of liquid 14 within enclosure 10 is a function of the storage capability required for handling aerosol removed from the stream of gas.
"The upright side walls 12 of enclosure 10 are spaced transversely outward from the side walls 17 of the container 16. This permits flow of liquid into the container 16 from all sides through the openings 27 and permits the liquid exiting from container 16 to spill about its entire periphery.
"The specific example of the scrubber utilized cylindrical side walls about the container 16, arranged vertically and centered about a vertical inner axis along the center of the illustrated inlet duct 23. The duct 23 was constructed as a straight vertical tube coaxially centered within the bed 21 along the vertical container axis."
The Postma U.S. Pat. No. 4,432,777 proceeds at column 4, line 56 et seq. to discuss a number of tests which were conducted using various sodium-compound aerosols to measure aerosol removal efficiency and using water as the wash liquid within the enclosure 10. Further, the flow rate of the gaseous stream was varied during each test and aerosol samples were periodically taken from both the inlet duct 23 and the outlet duct 24. Average and overall efficiencies of removal then were calculated; the reported data results indicated that the granular sizes within the bed 21 had no effect on the collection efficiency but that reducing the bed height by a factor of 2 increased aerosol penetration by a factor of 7. Overall and average efficiency calculations ranging from 96.78% up to 99.97% for the structure of FIG. 1 are reported.
FIG. 2 of the Postma U.S. Pat. No. 4,432,777, also reproduced as FIG. 2 herein, discloses a modification of the Postma structure of FIG. 1, reported to have achieved increased removal efficiency by combining the hybrid scrubber of FIG. 1 with an available fibrous filter shown at 28 in FIG. 2, remaining elements of the structure of FIG. 2 being identical to and being identified by identical reference numerals as those in FIG. 1. The fibrous filter unit is reported to have measured 0.61 m OD by 0.56 m ID by 0.61 m long. The gas leaving the bed 21 is described as having flowed upwardly into the central region of the fiber unit 28 and then horizontally through the fibrous materials before proceeding radially outwardly, as indicated by the horizontal arrows, to be exhausted through the outlet 26.
The Postma U.S. Pat. No. 4,432,777 further notes, beginning at column 25, line 54: "Hydraulic tests were performed without aerosols for various bed configurations to measure pressure drop, water circulation rate, and water level effects. Pressure drop through the porous bed 21 was found to be independent of gas flow rate at rates between gas superficial velocities of 0.002 to 0.504 m/s. The pressure drop through the apparatus was found to be primarily due to the static liquid head at the submerged open bottom end 24 of inlet duct 23. The internal water circulation rate was found to be a function of the gas flow rate, bed depth, granular size and inlet duct submergence. In checking water flow rate versus gas flow rate for various bed parameters using a granular rock bed, water was found to be pumped at a decreasing rate as the water level dropped until the level was down to about one half the bed depth. The test parameter having the greatest effect on water flow rate was the depth of submergence of the inlet duct 23.
"The present apparatus is capable of handling a gaseous stream at pressures of 10-50 psi, which are typically containment pressures for vessels utilized in nuclear reactor installations. Gaseous streams vented from such containment vessels may be throttled as necessary in order to meet flow rate limitations of a particular scrubber apparatus. No other pumping of the gaseous stream is required, thereby eliminating any energy requirements for activation of the scrubber.
"The tests conducted on the experimental model indicate that a passive self-cleaning aerosol scrubber can be designed based on a superficial gas velocity of 0.507 m/s and a bed depth of 0.608 m. The aerosol removal efficiency can be predicted to exceed 99% for the aerosols that might be expected in a nuclear installation. Aerosol removal efficiency would exceed 99.9% for all feasible particle distributions if a passive fibrous filter is included as indicated in FIG. 2.
"In this apparatus, an airlift is used to circulate wash liquid through the packing within porous bed 21. The packing is kept clean during use of the scrubber without requiring utilization of external liquid pumps, which would in turn require a source of energy. This is extremely important under those conditions where electric power is not available.
"Removal efficiency of the scrubber can be designed to have the value required for any particular application. A high removal efficiency for small particles can be realized. This is a distinct advantage over submerged tubes, where large bubbles lead to low removal efficiencies for finer particles.
"Because the porous bed 21 is continually wetted, trapped dust will be either dissolved or washed from the bed material. Therefore, large masses of airborne particles can be trapped without plugging the porous bed 21.
"As compared to a simple submerged tube, this apparatus has a much lower pressure drop in a device designed to yield the same removal efficiency. This results from the breaking of the gas stream into small parcels as it enters the porous bed 21.
"All of the flow paths through the porous bed 21, which are small in size, are washed by the liquid and therefore will not plug. The inlet duct, which is not washed, can be as large as desired to assure that plugging will be prevented. This is a great improvement over other bubble breakup devices, such as the use of small diameter submerged tubes."
While the device of the Postma patent thus is characterized as having very high removal efficiencies for aerosols, it acknowledges that a removal efficiency exceeding 99.9% for all feasible particle size distributions requires the use of the passive fibrous filter 28 of FIG. 2. Furthermore, to the knowledge of the present inventors, those of skill in the art having direct familiarity and experience with the filter of the Postma U.S. Pat. No. 4,432,777 considered it important that the level of liquid within the enclosure 10 be not greater than substantially the elevation of the top end of the container 16, as illustrated in each of FIGS. 1 and 2: as the patent itself discloses, the pumping function decreased as the water level was lowered relatively to the top of the bed and should not be less than approximately one-half the bed depth.
While the Postma hybrid filter affords many advantages and offers a solution to certain of the filtration requirements for venting containment gases during a postulated major accident, it has many limitations rendering it incapable of fully satisfying those requirements, at least for many nuclear installations.
A significant factor is that the efficiency which can be achieved from any filter, whether a pool-type liquid scrubber or a porous, e.g., sand or gravel, filter--i.e., the constituents of the Postma hybrid filter--is a function of the constituents of the vented gas, including, e.g., the type and size of aerosols, and particulate matter which must be removed. For example, the test results of the Postma filter are based on removal of sodium aerosols, which typically are in the range of 3 to 5 microns in diameter; however, many nuclear facilities involve aerosols of smaller sizes, such as cesium and iodine which are in the micron and sub-micron diameter dimensions. Necessarily, the removal efficiencies of the Postma filter as reported in the Postma patent would be reduced, for such smaller sizes of aerosols.
Even more critical limitations of the Postma filter are that it not only is incapable of providing any meaningful amount of decay heat removal, but also is incapable of functioning for the requisite time interval of passive response to the accident--typically, 24 hours. Specifically, the vented gases intended to be filtered by the Postma filter may be as hot as 1,000.degree. F.; that factor, plus the decay heat of the vented gases would produce rapid boiling and resulting vaporization of the water in the filter enclosure 10. As a result, the water would be inadequate, in volume, to achieve the necessary temperature reduction and decay heat removal, even proceeding from its initial condition, and would become increasingly deficient for these purposes as the water level decreases by boiling off. For the relative sizes of the enclosure 10 and the container 16 and for the specific examples thereof given in the Postma specification, the water level 15 would very rapidly descend below the half-height of the bed 21. Thus, not only would the filter have an inadequate volume of water even initially to effect decay heat removal, the pumping action essential to purging of the porous filter bed would diminish rapidly and eventually cease, as is alluded to in the Postma patent disclosure. Thus, the filtering function as well would be severely diminished.
Thus, in the postulated accident scenario, the Postma hybrid filter would fail adequately to provide the requisite filtered venting function, not only as to smaller particle size contaminants than the sodium aerosols, but also the requisite decay heat removal function, with or without the added fiberous filter, and its effective period of operation would be unacceptably short. It is significant in this regard that the intent is not only that the filter be operative after years of non-operating stand-by status, but also that no auxiliary power should be required to enable it to function. For the Postma filter to function for the requisite period of passive operation, e.g., 24 hours, would require that the vented gases be cooled preliminarily to minimize boiling of the water and its rapid evaporation--absent which, rapid lowering of the water level 15 and cessation of the pumping function, essential to the filtration function, would occur. Necessarily, cooling the escaping gases implies the use of some further equipment--which, if performing mechanical cooling, would necessarily imply the availability of auxiliary power. The need for auxiliary power, of course, is contrary to the required, wholly passive character of the apparatus to be relied upon for performing the filtered venting of gases from the containment structure in the event of a nuclear accident. Even if passive heat exchangers were employed to reduce the temperature of the vented gases, the Postma filter still would lack the ability to perform the requisite decay heat removal function.